Near field communication (NFC) readers, used for transmission of data to compatible NFC devices such as NFC tags and the like, typically include an antenna having at its input an antenna filter for filtering out unwanted frequencies in a signal to be transmitted. The reader may be required to transmit data at rates up to 848 kbps. However, the bandwidth of the antenna filter may be as low as 300 kHz, which for higher-rate data contravenes the Nyquist-Shannon criteria, and can lead to significant levels of intersymbol interference (ISI) in the transmitted signal.
Most known NFC readers are designed to support a transmit data rate of 424 kbps whilst also being able to generate a strong magnetic field at the reader's transmit antenna. These dual requirements are achieved by using an antenna filter with a Q factor that is high enough to achieve a minimum field strength but not too high to prevent data reception.
Part of a typical known NFC system is shown schematically at 10 in FIG. 1. In the system of FIG. 1 an NFC reader 12 comprises a power amplifier 14 whose output is connected to input terminals of an antenna 16 by means of an amplifier filter made up of resistors 18a, 18b and capacitors 20a, 20b which are connected in series between differential outputs of the power amplifier 16 and the input terminals of the antenna 14.
An NFC tag 22 communicates with the reader 12 by means of an antenna 24, with the other components of the tag 22 being represented by a capacitor 26 and a resistor 28 connected in parallel with the antenna 24.
The resistors 18a, 18b and the capacitors 20a, 20b are of fixed value, and the loaded Q factor of the antenna 16 (i.e. the Q factor of the antenna when it is coupled to the tag antenna) of the reader 12 is determined by the total series resistance of the resistors 18a, 18b (as well as the series resistance of connecting components). As the resistors 18a, 18b are of fixed value, the loaded Q factor of the antenna is a fixed value, although during operation of the reader 12 the Q factor of the reader is affected by the value of the load on the tag 22. The loaded Q factor of the antenna 16 has a value
      Q    =                  2        ⁢        π        ⁢                                  ⁢                  f          s                ⁢        L                              R          a                +                  R          b                      ,where fs is the frequency of the transmitted signal, L is the inductance of the antenna 16, Ra is the resistance of the series resistor 18a, and Rb is the resistance of the series resistor 18b. 